535. Encode and Decode TinyURL

Note: This is a companion problem to the System Design problem: Design TinyURL.

TinyURL is a URL shortening service where you enter a URL such as https://leetcode.com/problems/design-tinyurl and it returns a short URL such as http://tinyurl.com/4e9iAk.

Design the encode and decode methods for the TinyURL service. There is no restriction on how your encode/decode algorithm should work. You just need to ensure that a URL can be encoded to a tiny URL and the tiny URL can be decoded to the original URL.

class Solution {
public:
    Solution() {
        short2long.clear();
        long2short.clear();
        srand(time(NULL));
    }
    // Encodes a URL to a shortened URL.
    string encode(string longUrl) {
        if (long2short.count(longUrl)) {
            return "http://tinyurl.com/" + long2short[longUrl];
        }
        string randStr;
        for (int i = 0; i < 6; ++i) {
            randStr.push_back(dict[rand() % 62]);
        }
        int idx = 0;
        while (long2short.count(randStr)) {
            randStr[idx] = dict[rand() % 62];
            idx = (idx + 1) % 62;
        }
        short2long[randStr] = longUrl;
        long2short[longUrl] = randStr;
        return "http://tinyurl.com/" + randStr;
    }

    // Decodes a shortened URL to its original URL.
    string decode(string shortUrl) {
        string randStr = shortUrl.substr(shortUrl.find_last_of("/") + 1);
        return short2long.count(randStr) ? short2long[randStr] : shortUrl;
    }

private:
    unordered_map<string, string> short2long, long2short;
    string dict = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
};